Cosign CVE-2026-39395

Despite a moderate CVSS score of 4.3 (low integrity impact, no confidentiality/availability impact, unauthenticated, low complexity), this vulnerability poses significant real-world risk in supply chain and software verification contexts. The CVSS vector (AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N) indicates remote, unauthenticated exploitation requiring user interaction (reviewing and acting on false verification results). The core risk is not system compromise but trust degradation: a developer or CI/CD system relying on cosign's verification output could accept malicious or tampered attestations, leading to downstream acceptance of compromised artifacts. The logic flaw in predicate validation represents a fundamental trust mechanism failure rather than a minor integrity issue. No public exploit code or CISA KEV status was identified in the provided data, but the vulnerability directly undermines the assurance properties that cosign is designed to provide, making it a priority for organizations using attestation-based supply chain verification.

An attacker crafts a malformed attestation with an incorrect predicate type or invalid payload and submits it alongside a signed container image to a supply chain verification system or developer workstation running vulnerable Cosign. The developer or CI/CD pipeline runs cosign verify-blob-attestation on the malicious attestation, which incorrectly reports 'Verified OK' instead of rejecting the invalid predicate. The attacker's container or binary is then accepted into the trusted artifact pipeline, potentially distributing compromised code to downstream consumers who rely on the attestation verification.